Megaesophagus

Upon swallowing, food moves to the stomach via the esophagus or "food tube".
A weakening and dilation of esophageal muscles (megaesophagus) can be caused by
inflammation or obstruction. Megaesophagus usually appears in young puppies,
where it is congenital, or in older dogs. Megaesophagus is a serious condition.
When the esophagus is dilated or the muscles weakened, it can cause a dog to
regurgitate food before it reaches the stomach or even regurgitate it even
after it reaches the stomach, but has not not yet passed into the intestines.
Regurgitation can lead to aspirational pneumonia, a condition not to be
taken lightly.

Tasha began vomiting intermittently, which led to
esophagitis (irritation of the esophagus), which led to megaesophagus, which in
turn led to aspirational pneuomonia. Tasha's first regurgitation episode was
around noon on a Tuesday. The next day she was unable to keep water down and that
night was admitted to the ICU at Tuft's Veterinary Hospital. Nineteen days later
she went to the Rainbow Bridge.

Dog.Com
Comprehensive article on megaesophagus, including an overview of
the gastro-intestinal tract.